1. Cellular Wireless Networks Generally
Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are often referred to as “1xRTT networks” (or “1x networks” for short), which stands for “Single Carrier Radio Transmission Technology.” These networks typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Mobile stations typically conduct these wireless communications with one or more base transceiver stations (BTSs), each of which send communications to and receive communications from mobile stations over the air interface. Each BTS is in turn communicatively connected with an entity known as a base station controller (BSC), which (a) controls one or more BTSs and (b) acts as a conduit between the BTS(s) and one or more switches or gateways, such as a mobile switching center (MSC) and/or packet data serving node (PDSN), which may in turn interface with one or more signaling and/or transport networks.
As such, mobile stations can typically communicate with one or more endpoints over the one or more signaling and/or transport networks from inside one or more coverage areas (such as cells and/or sectors) of one or more BTSs, via the BTS(s), a BSC, and an MSC and/or PDSN. In typical arrangements, MSCs interface with the public switched telephone network (PSTN), while PDSNs interface with one or more core packet-data networks and/or the Internet.
Service providers have also introduced mobile stations and wireless networks that communicate using a CDMA protocol known as EV-DO, which stands for “Evolution Data Optimized.” EV-DO networks, operating in conformance with one or more releases and/or revisions of industry specification IS-856, such as Release 0 and Revision A, both of which are hereby incorporated herein by reference, provide high rate packet-data service (including Voice over IP (VoIP) service) to mobile stations using a combination of time-division multiplexing (TDM) on the forward link (from the network to mobile stations) and CDMA technology on the reverse link (from mobile stations to the network). Furthermore, some “hybrid” mobile stations can communicate with both 1x networks and EV-DO networks.
In the EV-DO context, a mobile station is typically referred to as an access terminal, while the network entity with which the access terminal communicates over the air interface is known as an access node. The access node typically includes a device known as a radio network controller (RNC), which is similar to a BSC in 1x networks. The access node also includes one or more BTSs, each including one or more antennas that radiate to define wireless coverage areas such as cells and sectors. Note that sectors are used in the balance of this written description as an example of a wireless coverage area, though this is for explanation and not to the exclusion of cells or other coverage areas. Among other functions, the RNC controls one or more BTSs, and acts as a conduit between the BTSs and a PDSN, which provides access to a packet-data network. Thus, when positioned in a sector provided by an access node, an access terminal may communicate over the packet-data network via the access node and the PDSN.
2. Reverse Noise Rise
In general, in a given sector, an access node can provide service to access terminals on one carrier frequency (i.e. carrier), or on more than one. Furthermore, interference can be, and often is, present on a carrier in a sector. As used herein, an instance of a given carrier in a given sector may be referred to as a sector-carrier. In general, on a sector-carrier, an access node receives transmissions from access terminals operating on that sector-carrier. However, the access node often also receives transmissions on that sector-carrier from other access terminals, other devices, and/or any other sources of interference on that frequency.
At a given moment, the sum total of what an access node is receiving on a given sector-carrier is known as the reverse noise on that sector-carrier. At regular intervals access nodes compute reverse noise rise (RNR), which is the difference between (i) the reverse noise that the access node is currently detecting and (ii) a baseline level of reverse noise. Thus, the access node computes how far the reverse noise has risen above that baseline.
To determine the baseline level of reverse noise, EV-DO networks typically periodically utilize what is known as a silent interval. During the silent interval, access terminals know not to transmit anything to the access node. The access node can then measure whatever else is out there. As such, the baseline may correspond to the amount of reverse noise when the sector-carrier is unloaded (i.e. without any transmitting access terminals). Other reverse-link-noise levels, such as 24-hour or other minimums, could also be used as a baseline.
In general, the lower the RNR is at a given moment, the more favorable the RF conditions are for communication between access terminals and an access node at that moment. Correspondingly, the higher the RNR, the less favorable the RF conditions are. Moreover, a low RNR generally corresponds to a sector-carrier being lightly loaded, in other words that is supporting communications for a relatively low number of access terminals. A high RNR, as one might expect, generally corresponds to a sector-carrier being heavily loaded, in other words that is supporting communications for a relatively high number of access terminals.
3. Reverse-Noise-Rise Control
Access nodes typically use the calculated value of RNR to, among other things, determine whether they should instruct access terminals on a carrier to reduce their reverse-link transmission power level. Generally, access nodes may transmit a message to all access terminals on the forward link indicating that the access terminals should reduce their reverse-link transmission power. However, a service provider may desire to regulate reverse-link traffic differently for different access terminals. For example, an access node may be configured to instruct only a subset of access terminals that it is serving to reduce their reverse-link transmission power under certain conditions and according to various criteria.
One specific way access nodes typically instruct access terminals to reduce their reverse-link transmission power level is to set or clear what is known as the Reverse Activity Bit (RAB), which is a value that the access node makes equal to 0 or 1, and repeatedly transmit the RAB to all the access terminals operating on a given sector-carrier. Note that making the RAB equal to 0 is known as “clearing” the RAB, while making the RAB equal to 1 is known as “setting” the RAB. Access terminals typically interpret a set RAB as an indication that they should decrease their reverse-link transmission power, and typically interpret a cleared RAB as an indication that they should increase their reverse-link transmission power.
With respect to how the access node typically chooses whether to set or clear the RAB, if the RNR is above a threshold (“RNR threshold” or “RAB threshold”), which is a configurable parameter that may be between 0 dB and 30 dB, the access node sets the RAB. If the RNR is below the RNR threshold, the access node clears the RAB. The access node transmits the RAB in a TDM channel—known as the reverse-activity channel—on the forward link. That channel is itself a TDM portion of a forward-link channel known as the Media Access Control (MAC) channel. The initial release of IS-856 is referred to as Release 0 (Rel. 0), while a subsequent revision is referred to as Revision A (Rev. A). Note that some EV-DO networks may provide both Rel. 0 and Rev. A service; that is, a given EV-DO network may provide service to access terminals that operate according to Rel. 0, and also to access terminals that operate according to Rev. A.